1. Field of the Invention
This invention relates generally to pyroelectric sensors and, more particularly, to a method of determining the polarization state of a pyroelectric element by applying an AC signal to the element and calculating, multiple times, the hysteresis loop switching energy of the element for a predetermined time frame, and comparing it to a reference switching energy at a corresponding reference temperature.
2. Discussion of the Related Art
A certain class of sensors make use of ferroelectric materials, and their pyroelectric effect for detection of temperature change. Sensors of this type have a wide range of applications, such as imaging in low visibility conditions, for example, poor weather conditions, night vision, etc. A ferroelectric material is a dielectric material that has a temperature dependent spontaneous electrical polarization in the absence of an externally applied electric field which can change state with the application of a critical field, where the polarization magnitude and direction within the ferroelectric material is identifiable by a hysteresis loop. The orientation of the polarization of the material can be changed by applying a reversing external electric field to the material. The electric dipoles within the material, that identify the orientation of the polarization, change when the external field is applied and in a proper circuit layout produce a hysteresis loop. Since spontaneous polarization is generally temperature dependent, ferroelectric materials can employ the pyroelectric effect for temperature detection purposes.
Any area of the hysteresis loop, either the entire saturated hysteresis area or merely a region of operation anywhere within the full loop, is representative of the switching energy required to change the polarization states of some or all the dipoles which make up the atomic lattice structure of the material at a given temperature for the specific state of excitation. Any change in radiation incident on the ferroelectric material, if absorbed, changes the temperature, and thus changes the associated loop area. FIG. 1 shows two charge versus voltage hysteresis loops for a particular ferroelectric material at a first temperature T.sub.1 and a second temperature T.sub.2. If plotted independent of physical dimensions, the magnitude of an externally applied alternating electric field is given on the horizontal axis and polarization, in charge density, is given on the vertical axis. The area of the charge versus voltage hysteresis loop of a ferroelectric material has dimensions of energy, and the loop area is a direct function of its temperature. The magnitude of the polarization changes with a change in the temperature of the ferroelectric material for a given electric field. A careful review of the two hysteresis loops in FIG. 1 will show that for the two different temperatures T.sub.1 and T.sub.2, the area within the loop is different. Consequently, an electrical measurement of the change in area anywhere within the major loop is an electrical signal corresponding to the change of the temperature of the material, and thus of the incident infrared radiation. The effect is of a dynamic nature due to the switching between polarization states of the pyroelectric material, and therefore, when measuring incident radiation, it is necessary to shutter the radiation, to reference the ferroelectric spontaneous polarization before each window opens to the scene.
Heretofore, all of the known ferroelectric/pyroelectric sensors that convert varying radiation energy to usable electrical signals greater than the inherent ambient noise of the sensor system operate in a passive mode. This means that the pyroelectric element operates at a given polarization state which is a function of temperature change, without any deliberate electrical polarization reversal. More specifically, passive pyroelectric detection only interrogates the polarization state of the ferroelectric material typically by measuring the net voltage across a poled capacitor structure, or by small signal AC excitation to determine the permittivity of the material (which is a function of the poled state), or some combination of these two methods. The practice in the industry to compare ferroelectric/pyroelectric sensors has been to measure the pyroelectric coefficient p, which is defined as the partial derivative of the displacement D with respect to the temperature T, p=(.DELTA.D/.DELTA.T) at a given bias field, E.sub.B. What this means is that for a physical geometry having sensor area A, the amount of coulombs of charge Q is generated per temperature T, and the pyroelectric coefficient p is expressed as: p=(1/A) [.DELTA.Q/.DELTA.T]. Unfortunately, this technique only represents a single cycle around a minor portion of the available signal energy as represented by the hysteresis loop area.
FIG. 2 shows a schematic block diagram of a known pyroelectric sensor system 10 that employs a conventional passive charge generation technique to determine the output of the sensor element. The sensor system 10 includes a chopper 12 that selectively gates radiation from a scene onto an infrared absorber 14 that is part of a pyroelectric element 16. The pyroelectric element 16 is made of a ferroelectric material that exhibits hysteresis loops which vary with temperature as shown in FIG. 1, and represents a single pixel element of the sensor system 10 that combines with other pixel elements (not shown) to generate an image, as is well understood in the art. The discussion herein is directed to an infrared imaging system, but as will be appreciated by those skilled in the art, sensor systems of this type are applicable to detect other wavelengths of radiation, including millimeter waves and microwaves.
The chopper 12 selectively blocks and passes the radiation directed to the pyroelectric element 16 at a predetermined frequency so that the pyroelectric element 16 sees a reference temperature when the chopper 12 is closed, and sees the temperature of the scene when the chopper 12 is open. The difference between the reference temperature and the scene temperature alters the shape of the hysteresis loop as shown in FIG. 1. The change in charge Q(t) 18 for the two loops is measured separately as a voltage across a sampling capacitor 20 and an amplifier 22, in a manner that is well understood in the art. Because no external electric field is applied to the pyroelectric element 16, the measured charge of the pyroelectric element 16 that charges the capacitor 20 for the two loops is the charge Q(t) where the hysteresis loop crosses the positive vertical axis for temperature T.sub.1 and the charge Q(t) where the hysteresis loop crosses the positive vertical axis for temperature T.sub.2. The sampling capacitor 20 stores the charge from the pyroelectric element 16 only each time the window is opened by the chopper 12. The effective pyroelectric coefficient p for this design is given as: EQU p=(1/A)[Q.sub.1 -Q.sub.2 ]/[T.sub.1 -T.sub.2 ]
In an alternate known design, the small signal level capacitance, i.e. (change in local slope of the Q versus V curve of either a poled or unpoled ferroelectric material) between the charge stored by the pyroelectric element 16 is measured for temperature T.sub.1 and T.sub.2 and then compared. FIG. 3 shows a schematic block diagram of a sensor system 26 including the chopper 12, the infrared absorber 14, the pyroelectric element 16 and the amplifier 22. Sometimes small bias voltage is applied to the pyroelectric element 16 from a bias source (not shown), and a capacitance meter 28 is used to measure the change in capacitance between the location on the hysteresis loop for both temperatures T.sub.1 and T.sub.2 relative to the bias voltage. Even though a small bias voltage is applied to the pyroelectric element 16 in this design, the mode of operation is still passive because the small bias voltage does not alter the polarization state of the ferroelectric material in any way, but merely measures its change in local permittivity as measured by a change in capacitance. The effective pyroelectric coefficient p is given as: EQU p=[(V.sub.rms)/A](.DELTA.C/.DELTA.T)
As is apparent, this detection scheme utilizes only a small portion of the hysteresis loop, and therefore the sensors are limited in their ability to differentiate signal from noise. Both of the techniques discussed above are dependent upon the condition that the ferroelectric material is left resident in one of its two spontaneous polarization states P.sub.s (+ or -), or some intermediate state thereof. The ability to measure the power from the pyroelectric element 16 between the temperature changes gives the sensitivity of the system. Because the signal-to-noise ratio is relatively low for the prior art sensors, this establishes the sensitivity of the entire system. Robust and relatively expensive system components, such as the chopper 12 and the amplifier 22 cannot increase the signal from noise, but only can prevent further degradation.
What is needed is a ferroelectric/pyroelectric sensor that measures more of the available signal energy from the hysteresis loop output from the pyroelectric element to provide a better signal-to-noise ratio than is currently available in the prior art sensors. It is therefore an object of the present invention to provide such a sensor.